The military said it would not cease all combat operations, but would stop shooting to allow civilians to get out as it did for a UN convoy on Thursday that carried out 226 seriously wounded people.

The Sri Lankan government pulled out of a Norwegian-brokered truce with the Tigers a year ago, and has since been battling to dismantle the rebel's de facto mini-state in the north.

Following months of heavy fighting, government troops have captured the Tiger's political capital of Kilinochchi and, later, the Tamil Tiger bastion of Mullaittivu on the northeast coast.

Government forces say they have confined the Tamil Tigers into a 300 square kilometres pocket of territory in Mullaittivu district.

Humanitarian relief organisations say that about 250,000 civilians are still trapped in the rebel-held area.

Tony Birtley, Al Jazeera's correspondent, reporting from Sri Lanka, said that in general civilians did not appear to be fleeing the fighting.

"The military are saying they're not coming because the Tamil Tigers are refusing to allow them out, that they are using them as a human shield. There are even stories of mines being put around their settlements to stop them from leaving," Birtley reported.

"Of course, the Tigers say that people are scared to come because they may be abused by the army, they may be killed by the army and they'll almost certainly go into a camp to be interrogated by the army.

"It's very unclear why [civilians are not leaving], but these people have spent a long long time living under the Tamil Tigers ... and there's a lot of distrust."

'Safe passage'

In an appeal published on a government website, Mahinda Rajapaksa, Sri Lanka's president, said the Tigers were endangering the civilians' lives by refusing to let them flee.

"I urge the LTTE, within the next 48 hours, to allow free movement of civilians to ensure their safety and security. For all those civilians, I assure a safe passage to a secure environment," he said.

The military also accuses the Tigers of firing artillery from populated areas inside an army-declared no-fire zone with the hope of creating a crisis to build pressure for a truce.

The Tigers deny that and have continually accused the military of firing into the no-fire zone.

The International Committee of the Red Cross has said that hundreds of people have been killed or wounded in the fighting since last week.

The government says that the reported numbers are too high, but it has not provided an exact figure.

It also insists there have been "zero civilian casualties".

More than 70,000 people have been killed in Sri Lanka's civil war, which grew out of complaints by Tamils, who have suffered decades of marginalisation at the hands of successive governments controlled by the Sinhalese majority.